Mesoporous carbon spheres with programmable interiors as efficient nanoreactors for H2O2 electrosynthesis

The nanoreactor holds great promise as it emulates the natural processes of living organisms to facilitate chemical reactions, offering immense potential in catalytic energy conversion owing to its unique structural functionality. Here, we propose the utilization of precisely engineered carbon spheres as building blocks, integrating micromechanics and controllable synthesis to explore their catalytic functionalities in two-electron oxygen reduction reactions. After conducting rigorous experiments and simulations, we present compelling evidence for the enhanced mass transfer and microenvironment modulation effects offered by these mesoporous hollow carbon spheres, particularly when possessing a suitably sized hollow architecture. Impressively, the pivotal achievement lies in the successful screening of a potent, selective, and durable two-electron oxygen reduction reaction catalyst for the direct synthesis of medical-grade hydrogen peroxide disinfectant. Serving as an exemplary demonstration of nanoreactor engineering in catalyst screening, this work highlights the immense potential of various well-designed carbon-based nanoreactors in extensive applications.

6.The production of H2O2 disinfectant under neutral or acidic conditions is more practical, and this can be emphasized in the manuscript.7.There are many related catalysts with better performance in other reported works, which are not demonstrated in the Tables S3-S5.The authors should add them in the Tables.
Reviewer #2 (Remarks to the Author): In this paper, the authors demonstrated the mass transfer diffusion in the mesoporous hollow sphere model using FES methods in fluid fields, followed by experimental applications of this hollow structure in catalysis.Several mesoporous hollow carbon sphere nanomaterials were synthesized using SiO2 as a template with varied experimental sequences and timing.These were characterized using TEM, SEM, STEM with EDS analysis, BET surface analysis, XRD analysis, Raman spectroscopy, and XPS spectroscopy.Their varied structure also exhibited different electrochemical performances for the 2-electron oxygen reduction reaction to generate hydrogen peroxide in both alkaline and near-neutral electrolytes, with over 16 h stability in flow cells.The reaction mechanism was also analyzed using in situ Raman spectroscopy and local pH tests.Overall, this paper chose a good model to analyze the effect of mesopores on the 2-electron oxygen reduction, and the electrochemical performance was satisfactory.Publication is suggested.However, the reviewer does have some questions regarding the model, electrochemical analysis, and the mechanism discussion, which need to be addressed.
Firstly, and crucially, the authors selected a mesoporous hollow sphere as the standard model, where the holes are cylindrical and isolated from each other.In actual scenarios, however, the pores in the materials are predominantly interconnected, slit-shaped pores (see TEM, SEM, and STEM images in Figure 2, and the Type H3 hysteresis loops in the N2 adsorption and desorption isotherms in Figure 3).The reviewer questions whether the fluid field in these actual materials will mirror that in the models.For example, the fluid may flow through the interconnected channels on the surface without entering the hollow cores of the spheres.
Secondly, as mentioned, the N2 adsorption and desorption isotherms in Figure 3a exhibited an H3 hysteresis loop, which demonstrated slit-shaped pores, not cylindrical pores as the authors indicated.In both the isotherms and the pore size distributions, no pores from the hollow cores were found.Additionally, the BET surface area increased while the average pore size decreased as the diameter of the hollow core expanded.Does this indicate the hollow core is not accessible or it is made of a set of smaller pores?If that's the case, the model might require adjustments.Furthermore, the N2 adsorption decreased when the P/P0 value rose from 0.4 to 0.8 in Supplementary Figure 22, which does not make sense for N2 adsorption.
Thirdly, in Figure 4a, the onset potential varied for different MHCSx materials.As per the given assumption, the only difference among MHCSx materials is the pore size and thus, the mass transfer.Yet, onset potential is predominantly dictated by the intrinsic catalytic properties of the active sites.The reviewer wonders whether the active sites across the different MHCSx materials are consistant and if the divergence in electrochemical performance stems from differences in active sites rather than solely from mass transfer.The variability of active sites in mesoporous carbon materials, in comparison to microporous materials, was addressed by Bao, et al. in ACS Sustainable Chem.Eng.2018, 6, 1, 311-317.In Figure 4d, MHCSx exhibited varied Tafel slopes, which are unaffected by the mass transfer process.This suggests that the reaction mechanism differs across the MHCSx samples.
Fourthly, Figure 4g intriguingly reveals that H2O2 selectivity consistently rose as electrolysis progressed within the same electrolyte.What's the reason for this phenomenon?Typically, the ring current either remains constant or decreases due to pollutant adsorption on the Pt ring, necessitating cleaning of the Pt ring every few hours.If the ring isn't contaminated, why was there a need to replace the electrolyte in the experiment?Fifthly, in Figure 4i, Figure 4i showcases the disc current LSV of MHCSx materials in 0.1 M PBS, presenting two sets of limiting currents with distinct slopes.The reviewer questions if this indicates the presence of varied reaction mechanisms at different potentials.
Sixthly, based on the experimental design, variations in mass transport shouldn't impact the electrochemically active surface area, which could influence the active site concentration and limiting current.What caused this?Minors: Ag/AgCl is not a stable reference in alkaline solutions due to the formation of Ag2O, which alters the reference potential.

Reviewer #3 (Remarks to the Author):
This study investigates the effect of geometric and porous structures of mesoporous hollow carbon nanospheres (MHCS) on two-electron oxygen reduction reaction (2e− ORR) activity for H2O2 electrosynthesis.The authors conducted fluid dynamics simulations to prove the advantages of mesoporous hollow structures, which could be attributed to facile O2 supply as well as H2O2 emission.A set of MHCSs was prepared with carefully controlled structural properties, yet with similar surface oxygen functional groups, thereby enabling model catalyst study.It is demonstrated that MHCS0.5 exhibits the highest H2O2 electrosynthesis activity owing to its optimal mesostructures which enables fast diffusion.The structure-dependent diffusion behavior was tried to be proved experimentally, where however, some issues should be addressed during the revision.
Overall, this reviewer would like to recommend the publication in Nature Communications, but after major revision.Detailed comments are listed below.
1.The density of mesopore is presumed to have a strong effect on the fluid behavior.This effect can be shown and briefly discussed in the revised manuscript.
2. The mismatch between the BET surface area and double layer capacitance trends of MHNs also suggests the effective diffusion of MHCS0.5 compared to other MHCSs because high double layer capacitance requires sufficiently fast supply of electrolyte ions.The authors can add some discussion on the relation between them.
3. It is wondered whether the H2O2 selectivity difference between MHCS0.5 (the best) and MHCS-0.5 (the worst) is demonstrated in flow cell experiments.
4. The last experimental results where the MHCS0.5-basedelectrode contained the largest amounts of OH− ions (the side products of ORR in neutral solutions) among the samples are interpreted that MHCS0.5 catalyst had a poorer ability of exhausting products than the other samples, which is contradictory to the authors' claim.This reviewer guesses that this is just a result of faster electrocatalytic reaction with MHCS0.5 as a catalyst than the others.The authors should explain this and provide a time-current figure of the catalysts obtained in "Direct detection of the local pH changes on electrode" experiments.5. Similarly, the in-situ Raman results shown in Fig. 6h, where MHCS0.5 exhibited the highest signals for *O2 and *OOH among the samples, were consequences of higher activity of MHCS0.5 than the other samples.Therefore, the authors should reinvestigate the results and the related sentences need tonedown.Response: We acknowledge the reviewer's suggestion.As shown in Supplementary Table 1, the particle radius distribution of MHCSx ranges between 150-210 nm.To simplify the model and reduce computational effort, we uniformly utilized the 150 nm model for constructing hollow mesoporous nanoreactors with varying hollow ratios, without compromising the accuracy of the predicted outcomes.
As a model of the predictive mechanism, it is best to control only for variations in the degree of hollowing as the sole variable for elucidating the mechanism.Therefore, we have standardized all model radius to 150 nm.
To address this concern raised by the reviewer, we have included an explanation in both the "Finite element simulation" section and the main text (highlighted by yellow).
(Page 15, Line 27-30) "To simulate the synthesized MHCSx, MHNs were uniformly constructed using a particle radius of 150 nm and a mesopore channel of about 20 nm, with the proportion of the hollow as the sole variable.In this way, the impact of various d/r on fluid transport is effectively demonstrated at a standardized particle size, optimizing computational power usage without compromising the accuracy of the findings."(Page 3, Line 9-11) "To ensure systematic construction of the MHNs and conserve computational resources for investigating the regularity of the electrolyte fluid, we standardized the particle size to 300 nm while maintaining the principle of unique variability for different d/r or φ."

Comment 1-4:
ECSA can assess the intrinsic active sites of the catalyst or the accessibility of these active sites.The authors determined ECSA using double-layer capacitance.To illustrate the relationship between the catalyst's active sites and its catalytic function, it is advisable for the authors to examine ECSA and consider normalizing electrochemical activity or H2O2 partial current.
Response: This suggestion from the reviewer is highly insightful.To address this concern, we standardized the electrochemical activity and H2O2 partial currents based on the electrochemically active area in the revised manuscript (Fig. 4e).As illustrated in Fig. 4e, the normalized electrochemical activity, H2O2 partial current, and H2O2 selectivity at diffusion-dependent potentials show a positive correlation with the simulated inflow rate.Excluding the effects on accessibility for active site, it is evident that diffusive mass transfer contributes to the enhancement of ORR activity and the promotion of two-electron selectivity.We have updated the figure and adjusted the description accordingly in the revised manuscript (highlighted by yellow).(Page 9, Line 21-24) "Furthermore, we utilized ECSA to standardize the electrochemical activity and the H2O2 partial currents of MHCSx at diffusion-related potentials (0.6 V vs RHE), as illustrated in Fig. 4e.
Interestingly, the associated normalized currents and 2e -selectivity exhibit a positive correlation with the simulated electrolyte inflow velocity, providing additional evidence for the connection between the diffusion effect and the electrochemical performance on these MHNs."(Page 14, Line 24-28) "The ECSA was determined through the calculation of the Cdl using the following formula: Where the specific capacitance (Cs) of the electrode surface typically ranges from 20 to 60 μF cm -2 , we adopted 40 μF cm -2 in this work.AGCE refers to the electrode surface area." Comment 1-5: When employing the RRDE method to assess catalyst stability over an extended duration, the accumulation of H2O2 can impact the ring current.However, the curves in this study exhibit a consistent ring current.How did the authors address the issue of H2O2 accumulation?
Response: In continuous testing, the gradual accumulation of H2O2 leads to a slight increase in the ring current, resulting in a slight increase in selectivity.To maintain the consistency of the ring current, we changed the electrolyte every 3 hours and rapidly performed electrochemical scavenging of the Pt ring to remove the impact of the accumulated H2O2 during continuous operation.We have further discussed and added the corresponding explanation in the revised manuscript (highlighted by yellow).
(Page 10, Line 2-4) "During the continuous testing, the accumulated H2O2 might lead to a slight rise in the ring current and subsequently in selectivity.To maintain a consistent ring current, we replaced the electrolyte every 3 h and performed electrochemical cleaning of the Pt ring to remove any effects of the accumulated H2O2 during continuous operation." Comment 1-6: The production of H2O2 disinfectant under neutral or acidic conditions is more practical, and this can be emphasized in the manuscript.

Response:
We greatly appreciate this suggestion from the reviewer.In response, we have included a discussion on the practicality of producing H2O2 disinfectants in neutral condition in the revised manuscript (highlighted by yellow).
(Page 10, Line 38-41) "Notably, the electroproduction of H2O2 solutions in neutral electrolytes is particularly appealing due to its environmental friendliness, minimal corrosiveness, and potential for reduced electrolytic cell costs 72 .Additionally, neutral H2O2 solutions provide a sustainable and versatile option for practical applications, including the direct use of synthesized H2O2 in biochemical systems 73,74 ." (Page 11, Line 7-8) "Significantly, the medical-grade H2O2 solution produced under neutral conditions can be elegantly stored or directly employed for disinfection and sterilization purposes."

Comment 1-7:
There are many related catalysts with better performance in other reported works, which are not demonstrated in the Tables S3-S5.The authors should add them in the Tables.

Response:
In response to this concern raised by the reviewer, we have included recently reported relevant electrocatalysts in Supplementary Table S3
-S5.The tables and the literature added are listed below, highlighted by yellow.Comparison of electrocatalytic H2O2 production performance by RRDE technique in neutral electrolyte (pH = 7) of recently reported electrocatalysts.Supplementary Table.5.Comparison of electrochemical performance in the flow cell device with recently reported electrocatalysts.